1. Field of the Invention
The present invention relates to measuring and detecting coagulation and coagulation-related activities in fluids, particularly human blood. More particularly, the present invention relates to an improved activated clotting time test for citrated blood samples which provides accurate and reliable detection of coagulation-related activities regardless of the amount of time which has elapsed since the sample was drawn.
2. Description of the State of the Art
Blood coagulation is a complex chemical and physical reaction which occurs when blood comes into contact with an activating agent, such as an activating surface or an activating reagent. In accordance with one simplified conceptual view, the whole blood coagulation process can be generally viewed as three activities: platelet adhesion, platelet aggregation, and formation of a fibrin clot. In vivo, platelets flow through the blood vessels in an inactivated state because the blood vessel lining, the endothelium, prevents activation of platelets. When a blood vessel is damaged, however, the endothelium loses its integrity and platelets are activated by contact with tissue underlying the damaged site. Activation of the platelets causes them to become "sticky" and adhere together. Additional platelets then adhere to the activated platelets and also become activated. This process continues until a platelet "plug" is formed. This platelet plug then serves as a matrix upon which blood clotting proceeds.
If the chemical balance of the blood is suitable, thrombin is then produced which causes conversion of fibrinogen to fibrin, which forms the major portion of the clot mass. During clotting, additional platelets are activated and trapped in the forming clot, contributing to clot formation. As clotting proceeds, polymerization and cross-linking of fibrin serves as the permanent clot. Thus, platelet activation plays a very important function in blood coagulation.
A number of different medical apparatuses and testing methods exist for measuring and determining coagulation and coagulation-related activities of blood. These apparatuses and methods provide valuable medical information to an attending physician. For example, the information assists a physician in prescribing medication, predicting post-operative bleeding and prescribing various therapies. Some of the more successful techniques of evaluating blood clotting and coagulation are the plunger techniques illustrated by U.S. Pat. No. 4,599,219 to Cooper et al., U.S. Pat. No. 4,752,449 to Jackson et al., and U.S. Pat. No. 5,174,961 to Smith, all of which are assigned to the assignee of the present invention, and all of which are incorporated herein by reference.
Automated apparatuses employing the plunger technique for measuring and detecting coagulation and coagulation-related activities generally comprise a plunger sensor cartridge or cartridges and a microprocessor controlled apparatus into which the cartridge is inserted. The apparatus acts upon the cartridge and the blood sample placed therein to induce and detect the coagulation-related event. The cartridge includes a plurality of test cells, each of which is defined by a tube-like member having an upper reaction chamber where a plunger assembly is located and where the analytical test is carried out, and a reagent chamber which contains a reagent or reagents. For an activated clotting time (ACT) test, for example, the reagents include an activation reagent to activate coagulation of the blood. A plug member seals the bottom of a reagent chamber. When the test commences, the contents of the reagent chamber are forced into the reaction chamber to be mixed with the sample of fluid, usually human blood or its components. An actuator, which is a part of the apparatus, lifts the plunger assembly and lowers it, thereby reciprocating the plunger assembly through the pool of fluid in the reaction chamber. The plunger assembly descends by the force of gravity, resisted by a property of the fluid in the reaction chamber, such as its viscosity. When the property of the sample changes in a predetermined manner as a result of the onset or occurrence of a coagulation-related activity, the descent rate of the plunger assembly therethrough is changed. Upon a sufficient change in the descent rate, the coagulation-related activity is detected and indicated by the apparatus.
Certain discoveries have been made which contribute to a better understanding of the role of platelets in an ACT test. Such discoveries suggest that the activation of the platelets has a significant and previously unappreciated effect on ACT test results. While it has long been suspected that platelet activation contributes to total blood coagulation times, until fairly recently, there has been no technique available for confirming and quantifying the impact of platelet activation on ACT. U.S. Pat. No. 5,314,826 to Baugh describes an improved ACT test which includes a platelet activation phase to accommodate the effects of platelet activation. In the platelet activation phase an activating reagent is mixed with a sample of blood to be tested, then the mixture is gently agitated in such a manner and for a period of time sufficient to establish a predetermined and predictable contribution to the ACT from platelet activation. To evaluate platelet function, two simultaneous ACT tests (with different platelet activation phases) are performed, and the difference between the ACTs is indicative of the platelet functionality of the sample of blood. In a further improvement, described in U.S. Ser. No. 08/640,275, filed Apr. 30, 1996, the sample of blood is mixed with a chemical platelet activating agent to facilitate the participation of active platelets in the blood clotting reaction, thereby shortening the clotting time of the blood. If the platelets are inactive or not functioning normally, the activator will have minimal or no effect on the clotting time. Both U.S. Pat. Nos. 5,314,826 and 5,925,319 are assigned to the assignee of the present invention, and are incorporated by reference in their entireties herein.
Although previous apparatuses using the plunger sensing technique have proven generally satisfactory, the need for certain enhancements has been identified. Specifically, while these techniques can measure and detect coagulation and coagulation-related activities in a sample of blood, none are designed to reliably reflect the true in vivo coagulation conditions regardless of the amount of time which has lapsed since the blood sample was drawn. Most of the apparatuses currently available for monitoring platelet function are designed specifically for freshly drawn blood, or the test must be performed on a sample of citrated whole blood at a predetermined time after the blood is drawn, for reasons which will be discussed below. Citrated whole blood is whole blood collected in a medium containing a calcium chelating agent, such as sodium citrate (citrate). The whole blood is mixed with the citrate or calcium chelating agent when the sample is collected.
Calcium plays a significant role in the analysis of blood clotting. The act of drawing blood initiates clotting reactions, and unless something is done to stop the process, the clotting times have no diagnostic significance. The formation of a clot is a multi-step process and several of these steps require the presence of calcium ions. By removing the calcium ions, as is the effect when the blood is collected in citrate, the blood can be prevented from clotting. To reinitiate the clot-forming process, calcium is added back into the whole blood (recalcification). Many ACT tests, including many platelet function assays, can be performed on blood which has been collected into a medium containing a calcium chelating agent. A calcium chelating agent is a chemical which reacts with the calcium in such a fashion that the calcium can no longer function in blood coagulation. The most common chelating agent is a salt of citric acid (citrate), since it has the fewest side effects on the components of the clotting system. By collecting blood into a medium containing a calcium chelating agent such as citric acid, sample collection and the assay on the citrated sample can be separated by a time period of up to several hours.
Most hospitals and laboratories would like to be able to use citrated whole blood. The use of citrated whole blood means that the assay does not have to be run at bedside, and allows transport of the sample from the patient to a clinical laboratory for testing. Existing coagulation detection apparatuses can perform ACT tests on recalcified samples of whole blood collected initially in citrate or in some other calcium chelating agent, to thereby provide the convenience for delaying the assay from the time when the blood sample was collected. Unfortunately, for many individuals, there is a marked difference in results of an ACT test run at bedside and a recalcified ACT test on a citrated whole blood sample. In an ACT test performed on fresh drawn whole blood, the contribution of platelets is rate limiting. However, in a recalcified ACT test performed on citrated whole blood at a significant time after collection of the sample, some component of platelet participation has been activated. Consequently, the time component of coagulation required to form this activity is not reflected in the recalcified ACT test results because the activity has partially or fully developed. Thus, after approximately the first 30-60 minutes after drawing the blood sample in citrate, the coagulation component from platelet activity has concluded and, consequently, when the recalcified ACT test is performed, the amount of time for the activation is not included within the results of the ACT test. The recalcified activated clotting times of the citrated blood get shorter and shorter until activation is complete. The magnitude of this initial drop in the activated clotting time depends upon the elapsed time after drawing the sample, as well as the individual subject or patient from whom the blood was drawn. Platelets in a stored citrated sample become activated at a rate and to a degree which varies from individual to individual.
Thus, to obtain ACT test results which reliably reflect the true condition of platelets in vivo, the ACT test must be performed either at bedside approximately contemporaneously with drawing the blood sample or at a predetermined time after collection to extrapolate the patient's baseline (non-activated) clotting time. When the test cannot be performed at bedside, the time of draw must be specified on the sample tube, as well as instructions regarding the timing of the test(s). Not only does this time factor complicate the performance of ACT tests, but such tests are generally subject to variable results and inaccuracies because individual variations in collection and test procedures are introduced by the technicians processing the samples and conducting the tests.
A need therefore exists for an improved activated clotting time test which can accurately and reliably detect coagulation and coagulation-related activities in blood samples regardless of the amount of time which has elapsed since the sample was obtained. The ability to measure and evaluate platelet activation and function independent of the time interval between collection and testing is important, particularly when the blood sample must be transported to a different location for evaluation. At the present time, there is no known apparatus or method which is universally applicable to both fresh-drawn blood and citrated whole blood which has been stored for an unspecified amount of time.